1. Field of the Invention
The present invention relates to methods and apparatus for measuring the size and size distribution of small particles in suspension. More particularly, it relates to improvements in such methods and apparatus in which the particle size and distribution is at least partially determined by measuring the respective intensities of scattered polarized radiation in two mutually perpendicular planes.
2. Discussion of the Prior Art
Being able to characterize small particles in a suspension or spray in terms of size and size distribution is often useful in carrying out research, development and quality control in many industries. Laser diffraction, based on Mie scattering theory or Fraunhofer diffraction theory, is commonly used to size small particles (typically micron-sized particles and larger) on the basis of the light-scattering intensity patterns produced by the particles irradiated by a monochromatic beam of radiation. Laser diffraction is a particularly popular particle-sizing technique due to its relatively broad particle-sizing range, its reproducibility, and the speed of analysis it facilitates. The foundation of such analysis is based on the fact that each particle irradiated by a monochromatic beam produces a characteristic scattering pattern that is determined, in part, by the particle size. Generally, larger particles scatter more light than do smaller particles, and the intensity of the scattering pattern decreases with increasing scattering angle according to a characteristic periodic pattern of minima and maxima. As the particle size gets smaller, the overall intensity of the scattering pattern decreases, as does the contrast between the minima and maxima. For particles in the sub-micron size range, the angular scattering intensity contrast is very small, making particle size retrieval difficult. As the particle size approaches the wavelength of the irradiating energy, the scattering patterns are virtually indistinguishable, and particle sizing can no longer be effected by laser diffraction.
Due to the above-noted particle size limitation that is inherent in the laser diffraction technique for sizing particles, other methods have been used for sizing particles in the sub-micron range. One such method is known as the Polarization Intensity Differential Scattering or “PIDS” method. It makes use of the fact that sub-micron particles will scatter an incident, linearly polarized beam of radiation in such a manner as to produce a characteristic scattering pattern that is dependent not only on the wavelength of the irradiating beam, but also on the direction of polarization of the beam. Thus, the scattering pattern produced by a horizontally polarized beam (i.e., a beam polarized in a direction parallel to a scattering plane defined by the incident beam and the scattered beam(s)) will differ markedly from that produced by a vertically polarized beam (i.e., a beam polarized in a direction perpendicular to such scattering plane). By determining the difference in intensities of the two scattering patterns (i.e., one pattern produced by a parallel polarized beam and the other produced by a perpendicularly polarized beam) over a relatively wide angular range and at different wavelengths of the incident beams, the PIDS technique has been successfully applied in sizing submicron particles of various materials as small as about 40 nanometers.
According to the known PIDS technology, as described, for example, in the commonly assigned U.S. Pat. No. 4,953,978 to Bott et al., a vertically polarized beam (i.e., polarized perpendicular to the intended scattering plane) of radiation of a chosen wavelength irradiates particles of interest. The intensity of radiation scattered by the particles is measured at several scattering angles, preferably centered about 90 degrees with respect to the direction of propagation of the irradiating beam. The direction of polarization of the irradiating beam is then changed by ninety degrees so as to be horizontally polarized, (i.e., parallel to the scattering plane), and the scattering measurements are made again. This process is then repeated using beams of radiation at different wavelengths. The results of these sequential measurements are processed according to a known algorithm to provide a particle size distribution for the particles of interest. It may be appreciated that the accuracy of this process ideally requires that the sequential measurements be made from the same particles. Since the particles will have a tendency to move between successive measurements, the suspension is continually circulated during the measurement process to assure its homogeneity. Further, each of the several measurements is accumulated over a period of time. Thus, the PIDS process, while being capable of sizing particles too small to be detected by the more conventional laser diffraction method, tends to be relatively time consuming.